1. Field of the Invention
The present invention relates to the treatment webs including films, foils and similar sheet-formed objects. In particular, the present invention concerns a novel method for improving the barrier and printing properties of industrial webs such as papers, boards and polymer films.
2. Description of Related Art
The packaging industry has developed many kinds of functional surface treatments and coatings for packaging materials in order to improve the appearance and printing properties of the products. A further important aim has been to achieve barrier properties which prevent moisture, grease, oxygen and aroma compounds from penetrating into or out from the product packed in the packaging material and, thus, for example for protecting the foodstuff from deterioration caused by contact with oxygen and moisture.
Traditionally, the best known barrier structures comprise metal films, such as aluminium foils, laminated on papers, boards or polymer films. Nowadays, the aluminum foils are to an increasing extent being replaced by various polymer films, which have good barrier properties, but which are degradable in nature. Since no single polymer can provide the same resistance to gas and water penetration as aluminium, the polymer-based laminates typically comprise multilayered polymer structures. These laminates are rather expensive and a further problem related with the prior art is formed by the difficulties with pulping and recycling polymer coated papers and boards.
Various processes for coating paper with coating compositions that are pressed against smooth surfaces to provide an even surface of the coating are known in the art. Reference is made to EP-A 631 013. EP-A 778 71. WO 97/20691, FI-B 90366. EP-A 619 178, and U.S. Pat. No. 5,451,456, the contents of which is herewith incorporated by reference.
As an alternative to the above-mentioned approaches using integral films which are attached to the desired substrate, a newly developed barrier concept comprises plasma coating of polyester films with particulate SiO2. Also chemical plasma deposits for SiO2 are used. The typical coating thickness is about 150-500 A. For water vapour, reductions in the transmission rates of up to 99.94% have been achieved. However, this new technology is expensive and suitable for large production scale only based on high investment costs. Furthermore, it should be pointed out that vacuum and/or plasma or electron beam coated films are difficult to print on and special printing inks are needed for affixing ink particles or colours on the surface.
It is an object of the present invention to eliminate the disadvantages of the known methods and to provide a novel method of functionally modifying the surface properties of paper, board and polymer materials by providing them with a layer having increased smoothness and improved resistance to penetration of gases and liquids.
It is another object of the invention to provide articles of web materials having improved functional surface properties.
These and other objects, together with the advantages thereof over known processes, which shall become apparent from specification which follows, are accomplished by the invention as hereinafter described and claimed.
The present invention is based on the concept of forming the surface layer from coating pigments which are compacted so as to provide a dense packing on the web. Such a layer can be achieved by forming an aqueous pigment slurry into a layer and contacting said layer with a smooth, preferably impermeable surface while simultaneously drying the layer. Further, it is preferred to have a minimum Zeta potential of the pigments in the dried layer.
As a result of the invention, a web with a coating layer is provided, said layer comprising densely packed particles forming a smooth surface having a roughness of a maximum 1-3 xcexcm and having a permeability for water vapour of less than 5 g/m2/day.
By means of the invention considerable advantages are achieved. In particular, the invention provides a layer which has excellent barrier properties against water and oxygen. These layers can be combined with various paper and board products as well as with polymer films. The paper and board products are easily reclaimed and can be treated in a normal pulper, because they do not contain any metal foils or polymer films.
The dense particle packing formed against a very smooth steel or other metal belt has a very smooth surface. This smooth surface is ideal for printing or for secondary coating with polymers. The surface can also be made from polymeric materials and coated or surface treated so that the separation of coating particles will occur from the surface.
The use of a glue for transferring the coating layer makes it possible separately to cast the coating layer on a smooth surface and then reliably attaching it to the web surface. On the other hand, when a water-soluble polymer as a tie layer or a glue between the fibrous web and the surface coating makes it is possible to separate or to peel off the coating from the web during treatment in a pulper, which improves recycling of the fibrous matter of the web.